X THE EXCRETORY SYSTEM 207 



portion of the body. The renal arteries, of which there are 

 usually from four to six, enter the kidney at the median edge 

 or near the latter on the ventral surface. The divisions 

 of the renal arteries are distributed to the renal tubules 

 (arteriae rectae), and also to the glomeruli (vasa afferentia). 

 The renal portal vein runs along the dorsal surface of the 

 kidney very near the outer margin. From the transverse 

 branches of this vein, which extend across the dorsal surface, 

 small vessels are given off which penetrate the substance of 

 the kidney and form capillary networks around the renal 

 tubules. The vasa efferentia, which emerge from the glo- 

 meruli, together with the efferent veins arising from this capil- 

 lary network, go to form the beginnings of the renal veins 

 which convey the blood from the kidney to the posterior 

 vena cava. The glomeruli are supplied only with arterial 

 blood, while the renal tubules receive blood from the renal 

 portal veins, and also, although to a less extent, from the 

 renal arteries. 



The function of the kidney is the elimination of waste 

 matters from the blood. The renal excretion, or urine, is a 

 fluid containing a large number of compounds in solution. 

 Most of the nitrogen leaves the body in the form of urea, 

 (NH2)2CO, which is a white crystalline compound, very solu- 

 ble in water. Urea represents the final product of the break- 

 ing down of the nitrogenous substances of the body, and it 

 has been shown that the formation of this substance takes 

 place to a large extent in the liver, from which it is given to 

 the blood by a process of internal secretion. The kidney 

 also excretes several salts such as the chlorides, sulphates, 

 and phosphates of sodium, potassium, calcium, and magne- 

 sium, and numerous other substances in smaller proportions. 



The specific roles of the glomeruli and tubules in renal 

 excretion has long been a matter of dispute. It is certain 



